A golf club is generally provided with a shaft, a head disposed at the top end of the shaft, and a aria disposed at the rear end of the shaft. When golfers hit a ball, the impact angle of the head face and the ball has a great influence on the direction of the hit ball. In order to send a ball in the direction desired by the golfers, it is important the ball hits the heed face at right angles.
The impact force applied to golf clubs at the moment of impact of the head and the ball is large. It has been considered until now that the impact force has the following influences on golf clubs. More specifically, it has been considered that the impact force causes torsional deformation in the shaft, so that the head face hits the ball at a vertically inclined angle relative to the ball, and thus the direction of the hit ball deviates from the direction desired by a golfer. Based on such an idea, various measures for suppressing the torsional deformation of the shaft have been proposed until now e.g., Patent Documents 1 to 4).